An active noise-reducing technology has been developed for a long time. It is possible to reduce, or even eliminate a noise generated from a noise source by an inverse sound source as generated by a speaker. The inverse sound source and the noise source are the same in their volume, but phases of sonic waves are completely contrary to each other, and thus, by wave motion's destructive interference principle, the noise may be reduced, even dispersed.
Since voice is a spherical wave motion, therefore, if reduction, even elimination of the noises in each position by the active noise-reducing manner is desired, it is often necessary to locate the noise source and the inverse sound source at the same spatial position, so that an effect may be achieved. If the position of the inverse sound source is different, the sonic waves' phases may not be counterbalanced completely, i.e. some position may occur cancellation interferences. The other positions may occur additive interferences.
However, the noise source and the inverse sound source usually locate at different spatial positions, therefore, the current active noise-reducing technology is only useful to reduce the noise at some specific spatial positions, incapable of reduce the noises in the whole space.
Such a technology is employed for reducing the noise generated by the fan's rotation. Such a noise is mostly generated from a turbulent flow caused by fan body's tail flow, in a conventional technology, the magnetic force is created to oscillate the fan body for generating another inverse sound source, so that it allows the noise source and the inverse sound source to be located at positions nearby each other, almost at the same position, and the active noise-reducing effect may be achieved.
However, it has been found that the employment of magnetic force for oscillating the fan body for generating an inverse sound source, taking an axial-flow fan, such as 105 mm×105 mm×32 mm, as an example. When the noise source's frequency is lower than 1000 Hz, in addition to resonant frequency of several fan body's structure, such a technology has a poor and irregular efficiency in conversion of electrical energy to sonic energy, and thus, it may be hard to use such a structure for effectively reducing fan's low-frequency noise.
The information disclosed in this “BACKGROUND OF THE INVENTION” section is only for enhancement understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known to a person of ordinary skill in the art. Furthermore, the information disclosed in this “BACKGROUND OF THE INVENTION” section does not mean that one or more problems to be solved by one or more embodiments of the invention were acknowledged by a person of ordinary skill in the art.